A Bluetooth radio system employs an Industrial Scientific and Medical (ISM) frequency band of 2400-2483.5 MHz. To avoid interference with other systems employing higher and lower frequency bands than the frequency band, the Bluetooth radio system uses the frequency band of 2402-2480 MHz without 2 MHz after 2400 MHz and 3.5 MHz before 2483.5 MHz, that is, uses 79 channels in total. The ISM refers to the frequency band allocated for industrial, scientific, or medical purposes. Since the ISM needs not to be permitted for the use of a radio wave, the ISM has been mainly used for personal radio devices to emit lower-power radio waves. For example, an amateur radio, a wireless local area network (LAN), or Bluetooth employs the ISM band.
Since several systems use the same radio band, radio wave interference may occur between systems. In order to prevent the radio wave interference, the Bluetooth employs a frequency hopping scheme. The frequency hopping scheme is a scheme to transmit a slight amount of packet (data) while rapidly moving many channels in a specific pattern. In the case of Bluetooth, the allocated 79 channels are hopped 1600 times per second.
Only when Bluetooth devices are synchronized with each other with respect to the hopping pattern, the Bluetooth devices may make communication with each other. In general, the Bluetooth devices are connected with each other while forming the relation of a master and slaves therebetween. If a master device fails to be synchronized with a slave device with respect to a frequency hopping pattern created by the master device, the communication between the master device and the slave device may not be made. Accordingly, when the master device and the slave device are synchronized with each other with respect to the frequency hopping pattern created by the master device, the master device and the slave device may be stably connected with each other while avoiding radio wave interference with other systems. For reference, one master device may be connected with up to 7 slave devices. The communication between the master device and the slave device is possible, but the communication between the slave devices is impossible.
Regarding conventional communication methods between short range radio communication devices, multiple devices may interfere with each other when the multiple devices mutually independently make communication.
For example, an audio video navigation (AVN) device, which is able to make short range radio communication, may make communication (first communication) with a first short range radio communication device and a second short range radio communication device, and a third short range radio communication device may make communication (second communication) with a fourth short range radio communication device. In this case, even though the first communication and the second communication employ the same frequency band, since the first communication and the second communication are mutually independent from each other, agreement may not made on the allocation of frequency slots. Accordingly, the first communication may interfere with the second communication.